1. Field of the Invention
The present invention relates generally to typewriters and printers and more particularly pertains to mechanisms for controlling the movement of thermal print heads for use in thermal typewriters and thermal printers.
2. Description of the Prior Art
Thermal printers and thermal typewriters have in the past included relatively complex structures to control the movement of the thermal print head toward and away from the platen and to apply a bias force for movement of the print head into engagement with the print medium or thermal ribbon which in turn engage the platen. Such control mechanisms have included coil springs and solenoids for providing biasing; and motors, solenoids, and electromagnets for overcoming such bias forces and releasing the thermal print head from contact with the recording medium.
Such structures are disclosed, for example, in U.S. Pat. Nos. 4,822,186, 4,844,632, 4,563,692 and 4,913,567. U.S. Pat. No. 4,822,186 discloses a complex mechanism employing a single reversible motor driving force coupled through a series of gears (transmitting means) for moving the thermal head to a non-printing position. The gears coact with a sector gear coupled to the print head for moving a lever against the bias force imposed by a coil spring. The reversibility of the motor enables movement of the thermal head between a printing and a non-printing position.
U.S. Pat. No. 4,844,632 discloses another complex mechanism which includes a biasing coil spring member which urges the print head against the platen by means of a transmission lever, a change lever and a pin, the latter of which engages a notch on the transmission lever. A print head release mechanism selectively relieves the spring bias force exerted on the print head and includes motor, a worm gear, a lever, an opening in the transmission lever and still a further spring means. There is also provided a mechanism for changing the level of bias force imposed on the print head.
The present invention discloses a typewriter or printer print head control mechanism for moving a print head from a print position proximate the platen to a non-printing position spaced from the platen. The control mechanism is mounted on a carrier housing which also includes a print head vertically supported by one arm of a bellcrank and whose other arm is coupled to a rotatable sector gear by a spring member A bi-directional stepper motor is coupled to the sector gear by a pinion gear for providing controlled rotary movement of the sector gear. The bellcrank is pivotable about a point proximate the intersection of its arms. The spring member is affixed to one end of the bellcrank arm and anchored to the sector gear proximate one of its side edges. The sector gear is rotatable about a center of rotation between the end of one of the bellcrank arms and the spring anchor, whereby upon rotation of the gear, the longitudinal axis of the coil spring member will cross over the sector gear center of rotation. When the coil spring longitudinal axis is on one side of the gear center of rotation, the spring is under tension and pivots the bellcrank to bias the print head against the platen. When the spring axis is on the other side of the gear center due to gear rotation, the spring is again under tension to pivot the bellcrank in the opposite direction to move the print head away from the platen. The mechanism further includes abutments for limiting the rotation of the sector gear.